Impedance measurement of an electrochemical cell including a plurality of electrodes and an electrolyte is widely used for, for example, clarification of a mechanism of an electrochemical reaction. As an impedance measurement method, an alternating-current impedance method for scanning a frequency of a sine wave signal applied to a measurement target electrochemical cell is known.
In the alternating-current impedance method, a frequency response analyzer (FRA) and a potentiostat are used. The FRA outputs a frequency response signal for applying a sine wave signal having a predetermined frequency to an electrochemical cell. The potentiostat controls, based on a frequency signal from the FRA, a voltage (an electric current) applied to the electrochemical cell.
By scanning the frequency having the sine wave signal, impedances at a plurality of frequencies, that is, a frequency characteristic of the impedance is acquired. A track of the impedance representing the frequency characteristic of the impedance in a complex plan view in which a Z′ (real number impedance) axis represents a resistance component and a Z″ (imaginary number impedance) axis represents a reactance component (usually, capacitive) is a Nyquist plot (a Cole-Cole plot).
The Nyquist plot shown in FIG. 1 is a case of a simple model that takes into account electrolyte resistance Rs, interface resistance Rint including charge transfer resistance and film resistance, capacitance C of an electric double layer incidental to the charge transfer resistance, the film resistance, and the like, and diffusion Zw of a charge carrier. That is, an electrochemical reaction of a simple system in an electrochemical cell in which a reference electrode is used is configured from movement of ions in an electrolyte, a charge transfer reaction on an electrode interface, and diffusion of the ions involved in the charge transfer reaction. Note that, in an electrochemical cell in which a reference electrode is not used, since impedances of two electrodes (a positive electrode and a negative electrode) are included, a locus of a semicircle is an overlapping locus of at least two semicircles. By analyzing the locus using an appropriate equivalent circuit model, characteristics of each of components such as a plurality of electrodes configuring the electrochemical cell and an electrolyte can be grasped.
For example, when the diameter of a semicircle indicating the interface resistance Rint increases, this indicates that a change occurs in the electrochemical cell. That is, this indicates that, in a secondary battery, the battery is deteriorated. When the electrochemical cell is a lithium ion secondary battery, it is surmised that resistance increases because of deterioration of an active material itself such as a change of a crystal structure and because a lithium ion electrolyte component and an organic solvent in the electrolyte decompose and deposit in a form of an organic substance and an inorganic substance on the surfaces of the negative electrode and the positive electrode as electrolyte decomposition products and insertion and desorption of the lithium ions are hindered.
An electric automobile and the like being spread in recent years uses a secondary battery, which is an electrochemical cell, as a power source. However, since costs increases, it is unrealistic to mount frequency characteristic analyzers and potentiostats in respective automobiles in order to evaluate characteristics with the alternating-current impedance method.
To realize a low carbon society, introduction of renewable energy such as solar power generation or wind power generation is in progress. In order to perform stable power supply using the renewable energy, a large power storage system is indispensable.
The large power storage system uses a large-capacity secondary battery as a main component. The large-capacity secondary battery has extremely low internal resistance. Therefore, to evaluate characteristics with the alternating-current impedance method, an extremely expensive large-capacity potentiostat is necessary. For example, when the internal resistance of the secondary battery is 10 mΩ, 300 A is necessary as a signal current of the potentiostat to control a voltage to 3 V. When the internal resistance is 1 mΩ, 3000 A is necessary as the signal current. Voltage control is not easy either.
Note that Japanese Patent Application Laid-Open Publication No. 2003-090869 discloses a measurement apparatus that applies, to a battery, a signal superimposed with sine waves having a plurality of frequencies and Fourier-transforms a response signal to acquire impedance at the plurality of frequencies.
Japanese Patent Application Laid-Open Publication No. 2012-185167 discloses that, in a power storage apparatus including a plurality of batteries, a pseudo sine wave signal is applied from one battery to the other batteries to measure impedance.
On the other hand, an embodiment of the present invention has an object of providing an electrochemical analysis apparatus having a simple configuration that can acquire impedance characteristics of an electrochemical cell and an electrochemical system including electrochemical analysis apparatus.